1. Field of the Invention
The present invention relates to a process and an apparatus for transferring a flux onto electrodes and a process and an apparatus for producing small metal bumps of a low melting point metal or metal alloy.
2. Description of the Related Art
The popularization of portable information devices and portable video cameras increases the need for miniaturized semiconductor packages. The advancing performance of LSI's also increases the number of electrodes. To achieve the miniaturized and multi-terminal packaging, a flip-chip bonding method is used to directly mount a chip on small-pitched electrodes of a substrate. A ball grid array (BGA) and a chip size package (CSP) have also been developed to realize a multi-terminal packaging, not requiring an extremely small pitch, by using a surface arrangement of terminals instead of a peripheral arrangement.
In this conventional technology, it is necessary to form connection terminals, or bumps, of a solder or other low melting point metal or metal alloy, on the electrodes of a substrate, etc. It is an advantageous bump forming method to form bumps by using balls of a solder or other low melting point metal or metal alloy. To form bumps from balls, in order to provide strong bond between the bumps and the electrodes, it is necessary that a flux is applied or transferred onto the electrodes, the balls are provisionally fixed to the electrodes, and the balls are then heated to above the melting point temperature thereof, thereby being melted to form bumps bonded to the electrodes.
The conventional technology has a problem in that, if a flux is also applied or transferred to portions other than electrodes, balls may flow down from the electrodes when melted by heating. This phenomenon can be prevented by transferring a flux only to the electrodes. U.S. Pat. No. 5,284,287 discloses a process comprising holding solder balls having a diameter of 500 to 700 .mu.m on an arrangement baseplate, moving the arrangement baseplate with the balls held thereon to above a flux bath, lowering the arrangement baseplate until portions of the balls are dipped in the flux bath to cause the flux to stick to the balls. The balls having the flux sticking thereto are pressed against the substrate electrodes to provisionally fix the balls to the electrodes. The flux is thus only applied on the electrodes to which the balls are provisionally fixed. The provisionally fixed balls do not flow down from the electrode when melted by heating.
This process of transferring a flux to electrodes only, however, becomes difficult to perform when the ball have a diameter less than 500 .mu.m. Specifically, when the balls have a smaller diameter, the distance between the arrangement baseplate which holds the balls and the tip portions of the balls to be dipped in the flux bath becomes smaller, so that the flux undesirably also sticks to the arrangement baseplate when the balls are dipped in the flux bath. When a baseplate having portions on which the flux is deposited is used again to hold another set of balls, the balls may also stick to the portions causing reduction in the reliability of the process of forming bumps on the electrodes. If the balls have a further smaller diameter, it becomes difficult to cause the flux to stick to the balls in only limited portions of the balls in a well-controlled manner. A problem then arises in that, if an excessive amount of the flux sticks to the balls, the sticking force causes the balls to drop off the arrangement baseplate into the flux bath.